The present invention relates to voltage regulators. More particularly, this invention relates to circuits and methods that provide the ability to adjust the minimum peak inductor current level and hysteresis for Burst Mode in current-mode DC-DC regulators (i.e. regulators that respond to measurements of output current or of signals indicative of output current).
Voltage regulators are power supply circuits that use a closed loop design to provide a predetermined and substantially constant output voltage, even while using an input voltage source which may be poorly specified or fluctuating. Furthermore, many electronic products use voltage regulators to convert an input voltage into a regulated output voltage that may be higher or lower than the input voltage. Accordingly, voltage regulators function as both a voltage converter in addition to a voltage stabilizer.
There are two major types of regulators: linear regulators and switching regulators. In a typical linear regulator, the output voltage is regulated by adjusting a passive element (e.g., a variable resistor) to control a continuous flow of current from the voltage source to the load.
Switching regulators, on the other hand, are essentially DC-DC converters that operate by switching current ON and OFF to control the output voltage. Switching voltage regulators typically employ one or more switching devices, along with an inductor and a capacitor in order to store and transfer energy to a load. These regulators are able to regulate the voltage being supplied to the load by turning the switching element(s) ON and OFF, thereby controlling the amount of power being transmitted through the inductor in the form of discrete current pulses. The inductor and the capacitor convert the supplied current pulses into a steady load current so that the load voltage is regulated. Ultimately, regulation of the output voltage is achieved through adjustment of the switch ON-OFF timings based on feedback signals indicative of the output voltage and load current.
Switching regulators that operate in current-mode are particularly desirable. They provide good line and load transient signal rejection, and possess inherent current-limiting capabilities during fault conditions (e.g., output short circuits). Many current-mode switching regulators monitor the inductor current and compare it with a peak inductor current level to determine when it is appropriate to turn OFF the main switching element, thereby eliminating the supply of excess current.
Many current-mode switching regulator circuits include the following: a logic section, an output switch or switches controlled by the logic section, an oscillator for providing periodic timing signals to turn ON the main switch, a current amplifier that relays a sense voltage that is dependent on the inductor current, an error amplifier that adjusts its output voltage depending on load conditions, and a current comparator that produces a signal that causes the logic section to turn OFF the main switch when the sense voltage compares in a predetermined manner with the voltage emerging from the error amplifier.
A particular type of regulator which often operates in current-mode as described above is the synchronous switching regulator. These regulators have a main switching element and a synchronous switching element which are driven out-of-phase with respect to each other in order to supply current at a regulated voltage to a load. Synchronous switching regulators differ from non-synchronous switching regulators in that a diode is replaced with a synchronous switching element, and the result, typically, is decreased power loss in the switching regulator.
A major benefit of switching regulators, such as synchronous switching regulators, is that they typically exhibit greater efficiency (where efficiency is defined as the ratio of the power provided by the regulator to the power provided to the regulator) than can be found in linear regulators, thereby leading to significant reductions in unwanted heat dissipation. As a result, many switching regulators can eliminate the use of a heat sink that an equivalent linear design would require.
In particular, synchronous switching regulators that employ MOSFET (metal-oxide semiconductor field-effect transistor) switches are widely used in portable battery-powered electronic products and products in which only limited heat generation can be tolerated. Because these voltage regulators exhibit higher efficiency, they provide relatively long battery life with little heat generation. For this reason, these regulators are often employed in systems such as cellular telephones, cordless telephones, personal pagers, laptop computers, and wireless modems.
The efficiency of switching regulators, however, is not always maximized and varies proportionally to the size of the load. It is a function of output current and typically decreases when the switching regulator is providing small amounts of current to the load. This occurs because even as the load decreases, a fixed amount of power is dissipated in the drive circuitry irrespective of the load size.
The above-described loss of efficiency at lighter loads is common in switching regulators that operate in a forced continuous mode of operation. In forced continuous mode, the efficiency loss at lighter loads for switching regulators becomes greater because the main switch is periodically turned ON and OFF regardless of operating conditions. Therefore, these regulators may become inefficient for smaller loads because of the energy, in the form of gate charge, that is required to constantly turn the main switch and synchronous switch ON and OFF regardless of load conditions.
An effective alternative to operating in forced continuous mode is to allow the regulator to enter Burst Mode operation. When operating in this mode, the regulator may omit switching cycles when the load is light, thereby reducing transistor gate charge losses. This is possible because, when operating in Burst Mode, the active switching elements (e.g., the switching transistor(s)) and optionally other unneeded portions of the regulator circuit are made to remain OFF as the load current drops below a specified value. The mode of operation when the active switching elements are made to remain OFF also is referred to herein as sleep mode. In a synchronous switching regulator, both the main and synchronous switching elements are made to remain OFF during sleep mode. In a non-synchronous switching regulator, only the main switching element is made to remain OFF. As one of ordinary skill in the art will understand, there may be different implementations of Burst Mode operation in different switching regulators. For example, different implementations may include different circuits and methods (1) to determine when a switching regulator enters sleep mode and/or (2) to charge the output capacitor. Burst Mode operation is used to reduce switching losses in a switching regulator and increase the operating efficiency at low output current levels.
Available regulators capable of operating in Burst Mode use essentially the same circuitry as described above for typical switching regulators, with the addition of a burst comparator and circuitry that provides a burst threshold level. This additional circuitry may be used to shut down part of a regulator circuit under specified conditions in order to reduce power consumption. Examples of regulators that use Burst Mode and forced continuous mode are marketed by Linear Technology Corporation, Milpitas, Calif., and include the LTC1435 and LTC1735 series products.
A disadvantage of some Burst Mode type regulators results from the inability to externally control the burst threshold level, which sets the minimum peak inductor current level. For higher burst threshold levels, the efficiency at light loads is higher at the expense of higher output voltage ripple (an undesirable characteristic). For lower burst threshold levels, the output voltage ripple is lower at the expense of slightly reduced efficiency for light loads. Accordingly, because the burst threshold level, which sets the minimum peak inductor current level, is fixed internally in current regulators that operate in Burst Mode, the output voltage ripple and efficiency of the regulator cannot be tailored to fit the requirements of different applications.
One way to adjust the output voltage ripple and efficiency of the regulator to fit the requirements of different applications is by externally setting the burst threshold level, as described in U.S. Pat. No. 6,724,174 to Esteves et al. This sets the minimum peak inductor current level, thereby permitting the output voltage ripple and efficiency to be adjusted to suit the requirements of various applications. At higher burst threshold levels, the efficiency at light loads would be higher at the expense of higher output voltage ripple (which, depending on the application, is an undesirable characteristic). At lower burst threshold levels, the output voltage ripple would be lower at the expense of slightly reduced efficiency for light loads. Externally setting the burst threshold level allows the switching regulator's output voltage ripple and efficiency to be tailored to the needs of a particular application.
Available Burst Mode regulators that permit external setting of the burst threshold level may use essentially the same circuitry as described above for regulators that use Burst Mode and forced continuous mode, with the addition of an adjustable burst clamp. This circuitry sets the minimum peak inductor current level according to the burst threshold level, which is set externally. Examples of Burst Mode regulators that use an adjustable burst clamp are the LTC3412, LTC3414, and LTC3418 series products marketed by Linear Technology Corporation, Milpitas, Calif.
In those Burst Mode regulators that use an adjustable burst clamp, the burst comparator hysteresis is fixed internally within the switching regulator. Since the only way to adjust the output voltage ripple amplitude and frequency in Burst Mode regulators using an adjustable burst clamp is by varying the minimum peak inductor current, the user has only one degree of control over the output voltage ripple and efficiency in tailoring them to fit the different requirements of each application.
Furthermore, for higher burst threshold levels, the efficiency at light loads is generally higher at the expense of higher output voltage ripple for those Burst Mode regulators that use an adjustable burst clamp. When the output voltage ripple exceeds a certain magnitude, however, there is a loss of efficiency in such regulators because of DC losses in the conduction path.
In view of the foregoing, it would be desirable to provide circuits and methods for permitting a user to externally set the minimum peak inductor current level.
It would also be desirable to provide circuits and methods for permitting a user to externally set the hysteresis of a Burst Mode comparator in switching regulators to increase efficiency of a regulator.
It further would be desirable to provide circuits and methods for permitting a user to more fully adjust the voltage ripple of the regulated output voltage of a regulator over a continuous range of values.